1. Field of the Invention
This invention relates to sag bending of glass sheets on bending molds and, in particular, to an apparatus and method for controlling the sag bending along selected portions of the glass sheets while moving the sheets on bending molds through a heating lehr.
2. Technical Considerations
In the practice of sag bending to form shaped glass windows for automobiles and the like, as disclosed in U.S. Pat. No. 4,375,978 to Reese, a glass sheet is supported on a skeletal bending mold. The shaping rail of the mold has a shape and configuration similar to that of the shaped glass sheet at a location slightly inboard of its peripheral edge. The bending molds are then conveyed in succession through a heating lehr where the glass sheet is heated to its deformation temperature such that it begins to sag by gravity until the glass sheet conforms to the configuration of the shaping rail. After the glass sheet is shaped, the mold is conveyed through an annealing zone where the glass sheet is cooled in a controlled manner from its deformation temperature through its annealing range to anneal the glass sheet. This gravity sag bending technique has been used to simultaneously shape two glass sheets, or doublets, which sheets are subsequently laminated together to form an automobile windshield.
As automotive stylists strive for more aerodynamic designs, the windshields are assuming more complex and deeper bend configurations, which are increasingly more difficult to form by conventional sag bending operations. It has been found that in producing shaped glass doublets with a deep curvature along its center line, because of the amount of heat and length of time required to achieve such a deep sag, portions of the glass in the vicinity of the shaping rails are prone to excessive sag bending.
Various arrangements have been developed to assist in controlling the shape of the glass sheets. In U.S. Pat. Nos. 4,265,650 and 4,894,080 to Reese et al., the glass sheets are pressed against one press face or between two opposing press faces. In U.S. Pat. Nos. 4,804,397 to Stas et al. and U.S. Pat. No. 5,049,178 to Pereman et al., partial presses are used to contact and press selected portions of the glass sheets. These shaping methods positively form the sheets to the desired configuration, but because the press faces contact the glass surfaces, there may be some marking. U.S. Pat. No. 5,167,689 to Weber controls bending at the corners of glass sheets supported on an outline mold by sag bending the sheets to a preliminarily curved configuration and subsequently lifting selected peripheral portions of the sheet off the curved shaping rails with additional shaping rails having the final desired elevational configuration. This arrangement reduces reverse bending at the corners of the glass sheets but does not address the problem of excessive sag of the glass near the shaping rails.
It would be advantageous to develop an apparatus and method for forming glass sheets while controlling excessive deformation of the glass in the vicinity of the peripheral shaping rails.